(1) Field of the Invention
The present invention relates generally to a capacitor and a method for making a capacitor. More particularly, this invention relates to a capacitor having a conductive layer with a plurality of microscopic trenchlike sections and a relatively simple method for making a capacitor having an increased capacitance.
(2) Description of the Prior Art
Remarkable progress has been made in the manufacture of Dynamic Random Access Memories (DRAM) using high integration technology. Accordingly, as a semiconductor device has a small size and the problem of securing a high capacitance becomes an issue. For example, main stream production has changed from 1 Mbit DRAMs to 4 M bit DRAMs has been achieved.
In such a DRAM with high integration, a predetermined capacitance of the cell storage capacitor must be kept constant, in spite of the decrease in the area of the cells. For example, each area of a cell and a capacitor in a 64 M bit DRAM decreases to about 0.8 .mu.m.sup.2 and 1.0 .mu.m.sup.2.
In the case where the areas of charge storage capacitors are also decreased and the capacitance becomes small, a soft error occurs on exposure to .alpha.-light, and the problem of reliability on a semiconductor device becomes an issue, too. Accordingly, the capacitance of cell storage capacitors must be kept constant, in spite of the decrease in the areas of the capacitors, in order to obtain an improved integration of a semiconductor device.
In a recent DRAM whose cells are based on transistor-stacked capacitor combinations, one of a pair of electrodes of a storage capacitor has been formed to have a three-dimensional structure. This makes the capacitance larger by 30 to 40% than that of a two-dimensional storage capacitor having the same size as the three-dimensional one. In the case of 64 M bit DRAMs having high integration, the capacitance needs to increase without the increase of cell areas or storage area, and various three-dimensional structures or high dielectric constant have been studied. One of technologies for increasing the capacitance of a storage electrode without the increase of cell areas or storage area is producing high-performance capacitors with a rough surface polysilicon film as a storage electrode.
A method for obtaining a high capacitance in defined small areas of capacitors, such as the above three-dimensional DRAMs, is described in "Solid state Device & Material No 90-167" page 49, published December 1990.
As a prior art, FIG. 1 shows a sectional view of the structure of a capacitor formed with a polycrystalline silicon film having a rough surface. A storage electrode 11 that serves as a first electrode of the capacitor, e.g. a first polycrystalline silicon film is deposited at 550.degree. C. At these temperatures, amorphous and polycrystalline structures coexist, and the surface areas of silicon grains having a hemispherical shape are maximized. Consequently the surface morphology strongly depends on the deposition temperature. In the surface area increase on 550.degree. C. deposited film, the area of the surface covered with hemispheres is about twice as large as that of the flat surface. After that, first electrodes are defined using conventional photolithography and etching technique. A capacitor dielectric film 12 of oxide film/nitride film is applied thereon, and a plate electrode 13 that becomes a second electrode, i.e. a polycrystalline silicon is deposited.
In the above method, however, since close attention to the control of temperatures should be paid and the polysilicon film has hemispherical grains, there is a limit on the increase of capacitance.